Seat belt retractors routinely have a belt reel which is journaled for rotation on a housing and a winding spring for urging rotation of the reel in the direction to wind the belt to a stored condition on the reel. The reel conventionally carries a toothed plate at each end thereof. A lock bar is pivotally mounted on the laterally spaced apart side walls of the housing and is movable by an inertia sensing mechanism into engagement with the sprocket teeth of the toothed plates to lock the belt reel against belt unwinding rotation.
In modern motor vehicles, it is desirable to make seat belt retractors as small and as light as possible. Thus, in the interest of reducing the size and weight of the retractor, it is desirable to reduce the diameter of the toothed plates which are carried by the reel and comprise a part of the retractor locking mechanism. As a result, when the belt is fully wound on the reel, the belt may be wound up to a diameter greater than the diameter of the sprocket teeth on the toothed plates so that the presence of the belt will block the movement of a lock bar into engagement with the toothed plates.
The locking mechanism employed in such seat belt retractors may be of the type including a sensing mechanism which is responsive to the rate of belt unwinding from the reel. Such a belt sensitive locking mechanism typically includes a pawl or weight which is eccentrically mounted on the reel and flies outwardly into engagement with clutch teeth of an annular clutch in response to the occurrence of a predetermined rate of belt unwinding reel rotation. The annular clutch is in turn coupled with the lock bar in a manner by which rotation of the annular clutch by the pawl will actuate the lock bar into locking engagement with the toothed plates carried by the reel.
The prior art has recognized that the teeth on the toothed plates carried by the reel and the teeth on the annular clutch should be equal in number and angularly related with each other so that the lock bar will fully engage with the sprocket teeth carried by the reel to carry the occupant restraint load and thereby prevent any further unwinding rotation of the reel and the pawl carried thereby so that the pawl and the annular clutch will not be subjected to the load occupant restraint.
The prior art has also recognized that the drive coupling arrangement between the annular clutch and lock bar may be provided by a split annular clutch ring which frictionally grips the annular clutch and will slip to forgive any delay in the locking of the lock bar with the sprocket teeth carried by the reel so that the pawl and annular clutch will not be overloaded. However, a disadvantage of such a slip clutch is that such slippage of the clutch upsets the angular relation between the clutch teeth of the annular clutch member and the sprocket teeth carried by the reel.